Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 100231

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
931 PM EST Tue Jan 9 2018

High pressure will crest over the region tonight before pushing off
the Mid Atlantic coast Wednesday. Low pressure will approach the
area late Thursday into Friday with a cold front passing through
Friday night into early Saturday morning. Canadian high pressure
then builds over the area Saturday night through Monday.


UPDATE 930 PM: Could see some patchy dense fog/black ice issues
mainly for southern MD and I-95 corridor near Fredericksburg VA.
Low confidence at the moment but may need to consider an SPS
later tonight based on how things evolve.

Surface high pressure is moving directly over the region this
evening providing light to calm winds. A scattered to broken
mid/upper cloud deck is preventing ideal radiational cooling
this evening, though. Temperatures are still dropping steadily
regardless. Expect high clouds to continue to increase overnight
causing temperatures to levels off after midnight. In addition,
warm air advection will begin aloft, with light return flow
developing, and this will likely lead to the formation of a
stratus deck for some locations west of the Blue Ridge towards
morning. There also remains the possibility for some light
rain/drizzle or freezing rain/drizzle, and although chances of
measurable precipitation are still quite low, even a trace of
freezing rain can create an issue. Will continue to highlight in
the HWO for Wednesday morning for locations west of the Blue
Ridge. There may also be some patchy fog, mainly across central
VA, southern MD, and near/along bodies of water.

Temperatures should fall at a decent rate after sunset thanks to
radiational cooling, but then will likely level out or even
gradually rise towards morning with increasing clouds. Forecast lows
are mainly in the 20s, except low 30s in the urban centers.


Once any drizzle/freezing drizzle ends during the morning hours
west of the Blue Ridge, a dry day is anticipated for much of
the area Wednesday. The ridge of high pressure that will then be
centered offshore will still be in control of the region`s
weather. We`ll probably be looking at mostly cloudy skies, but
some breaks of sun should still be around. Highs generally in
the 40s.

The ridge slides even further offshore by Wednesday night, opening
up the region to more return flow. Thus temperatures should be a bit
milder with lows holding in the 30s. More in the way of clouds are
also expected, and some fog/drizzle are even possible.

A low pressure system will then dig through the central/southern
Plains states on Thursday, amplifying an upper ridge over the
eastern US. This will act to increase southerly flow across the
eastern third of the US, and in turn, increase warmth and
moisture. A cloudy day is expected for Thursday, with some
showers becoming possible by the afternoon, especially
south/west. Highs in the low/mid 50s. First wave of rain out
ahead of the intensifying system will then overspread the region
Thursday night. Cannot rule out a rumble of thunder or two by
early Friday morning, but not enough confidence to place in the
forecast just yet.


With upper troughing situated to the west on Friday, deep layer
southerly flow will pump copious moisture into the region.
Precipitable water values will reach 1 to 1.5 inches. Thus,
would expect periods of rain to continue through the day.
Guidance currently has temperatures surging into the 60s across
most of the area, but would employ caution given potential for
low clouds and rain in the heart of winter. Low pressure will be
strengthening and lifting northward along or just west of the
Appalachians Friday night. Would expect additional rain/briefly
heavier showers to develop in association with a strong low
level jet. Some weak instability may develop, but it`s uncertain
if it will be deep enough to result in thunder. Forecast rain
amounts of 1 to 1.5 inches would not normally lead to concern
since it has been so dry recently, but we may have to keep an
eye on minor issues due to: (1) cold ground that is less
receptive to absorption, and (2) potential lingering ice on
streams and rivers. Hopefully the warm up over the next few days
will alleviate these issues somewhat.

As the low rapidly lifts toward interior New England early Saturday,
a cold front will sweep across the region, leading to clearing
skies, breezy conditions, and steady or falling temperatures (highs
in the 50s could occur early). An additional shortwave trough could
lead to upslope snow showers lasting through Saturday night,
although the rest of the area should remain dry.

Mean troughing will take hold over the eastern US through early next
week, which will bring a return to below normal temperatures (highs
in the upper 20s to mid 30s; lows in the teens and lower 20s).
Another shortwave trough may cross the area Sunday, but at the
moment it looks dry outside the mountains. Surface high pressure
will build across the area Sunday night into Monday. Weak low
pressure will be passing through the Great Lakes Monday night into
Tuesday, but it`s uncertain at this time if it will eventually bring
any precipitation locally.


UPDATE 930 PM: Have removed MVFR vsby at CHO (temp/dew point
spread looks too high), and also slowed MVFR cigs based on
latest trends/guidance.

There is potential for some low stratus development overnight
into Wednesday morning, with the greatest risk at CHO where a
broken MVFR deck was included in the TAF. This may spread
northward to MRB by later Wednesday morning. Some light
drizzle/freezing drizzle is also possible at CHO, but
probability is too low to include at this time. Potential for
patchy fog also exists, and have added MVFR visibilities to
IAD/BWI seems most likely. Cannot rule out fog elsewhere, or
patches of dense fog, but increasing clouds compared to last
night argue against that.

Mainly VFR is then expected Wednesday afternoon and evening, but
there is then increasing likelihood for sub-VFR conditions to return
Wednesday night through Thursday night with low stratus deck.

MVFR to IFR and possible lower conditions will be likely at
times Friday into Friday night due to occasional rain and low
clouds. Improvement will come Saturday behind a cold front,
although westerly winds may gust to 30 kt.


Sub-SCA winds are expected through at least Wednesday night with
high pressure overhead. Southerly flow will increase through the day
Thursday and especially Thursday night, along with the potential for
SCA conditions, though warm air over relatively cool water will
make mixing difficult.

Southerly flow will be strengthening Friday into Friday night as low
pressure moves west of the area. While the low level jet will be
strong aloft, winds may not mix well to the surface. Will have to
keep an eye on this period. A cold front will sweep through early
Saturday, with Small Craft Advisory conditions becoming likely
(possibly high end) in west to northwest flow. Winds should diminish
Saturday night into Sunday as high pressure builds west of the


Warming conditions are expected through the end of the week, along
with periods of rain Thursday night through Friday night. While dry
antecedent conditions should preclude any widespread hydrological
concerns, ice break up due to increasing warmth and rain may create
localized ice jam concerns. Confidence is currently too low and
localized to include in the HWO at the moment, but will monitor.
On average around one to one and a half inches of rainfall is
anticipated during this period, subject to change based on finer
scale details yet to be ironed out.


The recent brutal cold stretch has finally snapped. The cold peaked
during the first week of January, making it one of the coldest on
record for our area (and much of the eastern United States). Below
is a list of record coldest first weeks of January (1st through 7th).

Washington DC area (DCA)
1. 16.8 degrees (1918)
   16.8 degrees (1879)
3. 18.4 degrees (1877)
4. 19.0 degrees (2018)
5. 21.1 degrees (1877)
Temperature records date back to 1872

Baltimore MD area (BWI)
1. 15.2 degrees (2018)
2. 18.4 degrees (1918)
Temperature records date back to 1872

Dulles VA area (IAD)
1. 15.3 degrees (2018)
2. 19.5 degrees (1968)
Temperature records date back to 1960

Meteorological winter (which began December 1st) has also been very
dry so far. Below is a list of the top 5 driest meteorological
winters (December 1st through February 28th/29th) on record.

Washington DC area (DCA)
1. 2.60 inches (1871-72)
2. 3.32 inches (2001-02)
3. 3.85 inches (1980-81)
4. 4.15 inches (1976-77)
5. 4.76 inches (1873-74)

Notes: The winter of 1870-71 had 4.16 inches, but 31 days of data
are missing (no data for December 1870). So far, Winter 2017-18 has
had 0.66 inches through January 8th. Precipitation records date back
to 1871.

Baltimore MD area (BWI)
1. 4.03 inches (1976-77)
2. 4.12 inches (1980-81)
3. 4.28 inches (2001-02)
4. 4.30 inches (1871-72)
5. 4.51 inches (1979-80)

Notes: The winter of 1870-71 had 2.93 inches, but 31 days of data
are missing (no data for December 1870). So far, Winter 2017-18 has
had 1.07 inches through January 8th. Precipitation records date back
to 1871.

Dulles VA area (IAD)
1. 3.24 inches (2001-02)
2. 3.37 inches (1976-77)
3. 4.83 inches (1979-80)
4. 5.18 inches (1980-81)
5. 5.55 inches (2010-11)

Notes: The winter of 1962-63 had 0.00 inches, but 38 days of data
are missing. The winter of 1961-62 had 5.34 inches, but 34 days of
data are missing. So far, Winter 2017-18 has had 0.80 inches through
January 8th. Precipitation records date back to 1960.




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